The present invention relates to manganese-stabilized, austenitic stainless steel alloys suitable for use as bipolar plate materials for polymer electrolyte membrane (PEM) fuel cells, to bipolar plates comprising the alloys, and to PEM fuel cells comprising the bipolar plates.
PEM fuel cell assemblies frequently are constructed as stacks of multiple fuel cells. The individual fuel cells in the stacks each are separated by a flow field plate, of which one side functions as the anode for one fuel cell while the opposite side functions as the cathode for an adjacent fuel cell. Such a flow field plate, therefore, is commonly known as a bipolar plate.
Bipolar plates for PEM fuel cells must be electrochemically stable in an acidic fuel cell environment, must have sound mechanical properties, and must be easy to form into thin devices that have distribution channels on both sides of the plate. In addition, they must have a minimum contact resistance with the gas diffusion medium that is used to distribute the reactive gases to the catalyst layers inside the fuel cell. Minimum contact resistance typically is accomplished by choosing materials with high resistance to corrosion and low conduciveness to formation of passive layers. Importantly, bipolar plates must be cost effective so as to allow for mass commercialization of PEM fuel cells.
Metallic alloys in general and austenitic stainless steels in particular are considered potential material candidates for bipolar plates. Compared with bipolar plates comprising materials such as carbon composites, bipolar plates comprising alloys such as austenitic stainless steels offer advantages including decreased weight and increased volumetric power density. However, the cost of high-nickel austenitic stainless steels has increased considerably during the last few years, in part because of unprecedented demand for low cost stainless steels for commercial use in emerging markets such as India and China. Accordingly, there is a need to replace the expensive nickel component partially or completely in austenitic stainless steels to reduce cost and to allow for mass commercialization into PEM fuel cells.
Nickel is added to stainless steels to stabilize the austenite phase. If nickel is reduced or replaced in an austenitic stainless steel, other austenite stabilizers must be added to stabilize the austenitic structure. Carbon, copper, nitrogen, manganese, and cobalt are austenite stabilizers also, but the proportion of each added must be chosen with care. For example, carbon added as an austenite stabilizer can cause depletion of chromium around grain boundaries and thereby can sensitize the alloy to corrosion. Addition of copper under such circumstances can mitigate the sensitization to corrosion, but copper too is a relatively expensive metal. Manganese, on the other hand, is less expensive but is about half as effective as nickel for stabilizing the austenitic phase. As such, about twice the weight of manganese typically would be required in place of a given weight of nickel unless another austenite stabilizer such as nitrogen or copper were present.
Manganese austenitic stainless steels are known in the art. Standard grades of manganese austenitic stainless steels belong to the “200-series” alloys classified by the American Iron and Steel Institute (AISI). Alloys belonging to this series typically may contain low amounts of nickel. However, to retain an austenitic structure in steels with manganese substituted in the place of nickel, a corresponding reduction of chromium generally is necessary. For example, decreasing nickel content in an austenitic stainless steel from 12 wt. % to 1 wt. % may require reducing chromium content from 18 wt. % to about 15 wt. % to maintain an austenitic structure if manganese replaces a substantial amount of the nickel and all other components of the steel are kept the same. Because chromium is useful in promoting corrosion resistance in stainless steels of all varieties, it follows that the benefit of using less nickel can be outweighed by the detriment to corrosion resistance from having less chromium.
When manganese is added to an iron-chromium system such as a stainless steel, the solid and liquid solubility of nitrogen increases sufficiently to allow nitrogen to act as an effective austenite stabilizer. For example, when starting with an alloy such as 316 stainless steel with 18% chromium, and eliminating the 12% nickel that is normally present, roughly 12% manganese and 0.5% nitrogen will be sufficient to stabilize the austenite in place of the nickel without a corresponding need to reduce the amount of chromium. However, depending on the casting method for the alloy composition, excess nitrogen can cause formation of various iron or chromium nitride passive layers that are disadvantageous to contact resistance of the final alloy composition. Therefore, it is important to select a nitrogen content high enough to permit stabilization of the austenitic phase yet low enough to avoid undesirable propagation of nitride passive layers.
Examples of alternative austenitic stainless steels that have less Ni but rather appreciable quantities of other austenite stabilizers are 201 from Allegheny Ludlum (5.50-7.50 wt. % manganese and 3.50-5.50 wt. % nickel), Nitronic 30 from AK Steel (7.0-9.0 wt. % manganese and 1.5-3.0 wt. % nickel), and 204-Cu from Carpenter Technology (6.50-9.00 wt. % manganese, 2.00-4.00 wt. % copper, and 1.50-3.50 wt. % nickel). However, the uses of these steels have been limited to applications that require high strength but only moderate corrosion resistance. Moreover, the production of these alloys has not been scaled up to the same magnitude as nickel-based austenitic stainless steels, owing to low demand or to technical problems frequently encountered during production. Manganese-rich alloys are generally undesirable for use in highly corrosive environments, because even small amounts of sulfur present in the alloys can combine with manganese to form manganese sulfides that are known to function as active sites for corrosion.
A significant reduction in the material cost of stainless steel bipolar plates for PEM fuel cells is possible from the replacement of nickel with manganese as an austenitic stabilizer. Therefore, there is a need for low-cost, manganese-stabilized austenitic stainless steel alloys that can perform well in high-corrosion environments.